ZnCo2O4@g-C3N4@Cu as a new and highly efficient heterogeneous photocatalyst for visible light-induced cyanation and Mizoroki–Heck cross-coupling reactions

Conducting C–C cross-coupling reactions under convenient and mild conditions remains extremely challenging in traditional organic synthesis. In this study, ZnCo2O4@g-C3N4@Cu exhibited extraordinary photocatalytic performance as a new visible light harvesting heterogeneous copper-based photocatalyst in cyanation and Mizoroki–Heck visible-light-driven cross-coupling reactions at room temperature and in air. Surprisingly, by this method, the cyanation and Mizoroki–Heck cross-coupling reactions of various iodo-, bromo- and also the challenging chloroarenes with respectively K4[Fe(CN)6]·3H2O and olefins produced promising results in a sustainable and mild media. The significant photocatalytic performance of ZnCo2O4@g-C3N4@Cu arises from the synergistic optical properties of ZnCo2O4, g-C3N4, and Cu. These components can enhance the charge carrier generation and considerably reduce the recombination rate of photogenerated electron–hole pairs. No need to use heat or additives, applying an economical and benign light source, utilizing an environmentally compatible solvent, facile and low-cost photocatalytic approach, aerial conditions, high stability and convenient recyclability of the photocatalyst are the remarkable highlights of this methodology. Moreover, this platform exhibited the ability to be performed on a large scale, which is considered an important issue in industrial and pharmaceutical use. It is worth noting that this is the first time that a heterogeneous copper-based photocatalyst has been employed in visible light-promoted cyanation reactions of aryl halides.


Introduction
Nowadays, enhanced environmental consciousness has promoted the efficiency of the chemical processes under more benign and sustainable conditions, both in industrial and academic contexts.The main factors supporting chemical transformations toward green chemistry are using eco-friendly heterogeneous catalysts, safe and benign solvents and alternative energy sources. 1 In this context, the establishment of visible light harvesting-based photocatalysis has offered a technically attractive and energy-saving platform to effectually promote the chemical processes under mild conditions. 2 The use of visible light mediated strategies is highly recommended, since it is clean, abundant in the solar spectrum (44%), easy to access and accompanied with less side-reactions. 3,4raphitic carbon nitride (g-C 3 N 4 ) is a non-metallic semiconductor with photo-responsive attributes, making it a precious candidate for environmental remediation. 5,6Due to having the advantages of easy preparation, narrow band gap energy (2.73 eV), low-cost, chemical/thermal robustness and nontoxicity, it has been extensively investigated for visible light mediated photocatalytic processes. 7However, the rapid recombination of photo-excited electrons/holes in g-C 3 N 4 has limited its efficient photocatalytic performance. 7Incorporation of g-C 3 N 4 with other metal oxide semiconductors could effectually improve its visible light photocatalytic properties. 6s an affordable, environmentally friendly, and capable transition bimetallic oxide, ZnCo 2 O 4 has been reported to be a supreme candidate to make effective heterojunctions with other semiconductors. 8,9ZnCo 2 O 4 with the band gap energy of about 2.32 eV has specic optoelectronic features, resulting to decrease the rate of photogenerated electron/hole pairs recombination. 9Moreover, owing to the particular crystalline lattice and the synergistic effect of two metal species, ZnCo 2 O 4 shows superior photoelectrochemical stability and electron conductivity in comparison with single metal oxides such as Co 3 O 4 and ZnO. 10,11Inspired by this consideration, it is supposed that the band edge of ZnCo 2 O 4 could match appropriately with g-C 3 N 4 one.So, the constructed heterojunction composed of both semiconductors have the ability of enhancing the visible light absorption potential of the nal composite by slowing down the recombination velocity of the photo-excited electron-hole pairs. 83][14] Amongst them, copper NPs with a plasmonic nanostructure [15][16][17][18] have attracted increasing attention in photocatalytic developments owing to the stability, non-toxicity, low-cost, and availability. 19,20][17][18][19][20] For instance, the Enzalutamide (anti-prostate cancer), Letrozole (anti-breast cancer) and Rilpivirine (anti-HIV), which were amongst the top 200 brand-name drugs by retail sales in 2019, 21 possess nitrile motif in their composition (Fig. 1).
The conventional known approaches for the synthesis of aryl nitriles are consist of the Rosenmund-von Braun and Sandmeyer reactions. 22,23These procedures suffered from critical complications, such as the elevated reaction temperatures and needing the stoichiometric quantities of extremely toxic CuCN.Transition metal-catalyzed cyanation of aryl halides has evolved rapidly as an effectual alternative for the preparation of aryl nitriles in rather milder and benign conditions. 24,25Along this line, Pd as the most widely used transition-metal in crosscoupling reactions, [26][27][28] and a few examples of Ni, 29 Co, 30 Ir, 31 Rh, 32 and Cu based 33 catalytic systems with different cyanide reagents such as KCN, 34 CuCN, 35 NaCN, 36 CuSCN, 37 Zn(CN) 2 , 38 AgCN, 39 TMSCN, 40 etc., have been developed to perform the cyanation reactions.However, the high toxicity or sensitivity to the moisture of most cyanide sources seriously limited the industrial applications of these protocols.Currently, K 4 [Fe(CN) 6 ]$3H 2 O has become as the cyanide reagent of choice in cyanation reaction of different aryl halides.Compared with the conventional cyanide sources, K 4 [Fe(CN) 6 ]$3H 2 O is nontoxic, non-hygroscopic, convenient to use, cheap and commercially available.As an important issue, the predomination on severe affinity of metal catalysts for cyanide groups is a challenging problem that should be seriously considered in the cyanation reactions.Interestingly, the gradual release of cyanide ions from K 4 [Fe(CN) 6 ]$3H 2 O could effectually address this problem and hinder the catalyst deactivation.
][43][44][45][46][47][48][49][50][51] Among them, a few reports are related to the use of K 4 [Fe(CN) 6 ]$ 3H 2 O. [46][47][48][49][50][51] These procedures usually required harsh reaction conditions, rather expensive/toxic additives or ligands, hazardous solvents, prolonged reaction times and are associated with complicated product isolation and unsatised yields.A literature survey clearly pointed out that there is only one report for the light-assisted cyanation reaction of aryl halides using a copper-based photocatalyst. 52This procedure was accomplished using NaCN as a cyanide source.It has been performed under harmful UV-light irradiation and required very strict reaction conditions, alongside a complicated catalyst.Surprisingly, there is not any report on the visible lightpromoted cyanation reactions of aryl halides catalyzed by a heterogeneous copper-based photocatalyst.
Mizoroki-Heck cross-coupling reaction of aryl halides with olens is one of the most crucial routes for the carbon-carbon bond generation in synthetic organic chemistry.4][55][56][57][58] Fig. 2 shows the structures of some drugs prepared by Mizoroki-Heck cross-coupling reaction.
Notably, most of the methodologies developed for the Mizoroki-Heck reactions rely on using Pd-based catalysts, costly/ toxic solvents or bases, elevated temperatures and harsh reaction conditions.0][61][62][63][64][65] Unfortunately, most of these methods remain unaffordable and involve strict and complex photocatalytic reaction conditions.Hence, the introduction of a costeffective heterogeneous photocatalyst to proceed the Mizoroki-Heck cross-coupling reactions through a mild, efficient and environmentally compatible procedure has received signicant interest.

Paper RSC Advances
Scheme 1 Schematic representation for the preparation of ZnCo 2 O 4 @g-C 3 N 4 @Cu.
In continuation of our research interest to establish the new and efficient catalytic systems for sustainable and benign progress of cross-coupling reactions, 66-72 herein, we have introduced ZnCo 2 O 4 @g-C 3 N 4 @Cu 73 as a new and procient visible light-induced photocatalyst for conducting the cyanation and Mizoroki-Heck reactions using visible light irradiation at room temperature and in air atmosphere.

Results and discussion
Synthesis and characterization of ZnCo 2 O 4 @g-C 3 N 4 @Cu The photocatalyst was prepared based on the process shown in Scheme 1.Initially, g-C 3 N 4 was synthesized by sequential polymerization and liquid exfoliation techniques.Subsequently, the resulting g-C 3 N 4 was added to an alkaline mixture of Co(NO 3 ) 2 $6H 2 O and Zn(NO 3 ) 2 $6H 2 O, stirring under reux.Heating the obtained raw ZnCo 2 O 4 @g-C 3 N 4 sample at 350 °C, followed by reducing Cu(OAc) 2 on the surface of ultimate ZnCo 2 O 4 @g-C 3 N 4 afforded the desired ZnCo 2 O @g-C 3 N 4 @Cu.The freshly synthesized ZnCo 2 O 4 @g-C 3 N 4 @Cu was characterized by different techniques.

FT-IR spectra
FT-IR spectra of the pristine g-C 3 N 4 and ZnCo 2 O 4 @g-C 3 N 4 @Cu are presented in Fig. S1.† In the FT-IR spectrum of g-C 3 N 4 Fig. 4 UV-vis DRS of (a) g-C 3 N 4 , (b) ZnCo 2 O 4 @g-C 3 N 4 and (c) ZnCo 2 O 4 @g-C 3 N 4 @Cu.(Fig. S1a †), the indicative intense adsorption band at 808 cm −1 could be certied to the breathing mode of triazine units.The board adsorption bands located at 1200-1650 cm −1 could be ascribed to the stretching vibrations of C-N and C]N bonds.In addition, the broad adsorption band at about 3000-3400 cm −1 could be allocated to the stretching vibration modes of -NH bonds and the adsorbed water molecules.As it is obvious, the characteristic bands of g-C 3 N 4 could be easily detected in the FT-IR spectrum of ZnCo 2 O 4 @g-C 3 N 4 @Cu (Fig. S1b †).Besides, the appearance of a new distinct absorption band at 638 cm −1 is consigned to the stretching modes of Co-O bonds in the catalyst structure.

EDS analysis and elemental mapping images
EDS analysis of the ZnCo 2 O 4 @g-C 3 N 4 @Cu demonstrated the presence of O, N, C, Zn, Co and Cu elements (Fig. S2a †).Existence of these elements with homogeneous distribution on the entire surface of the catalyst in the elemental mapping images (Fig. S2b-h †), showed the uniformity of the elemental composition of ZnCo 2 O 4 @g-C 3 N 4 @Cu.

ICP-OES analysis
ICP-OES analysis was determined the copper content of the photocatalyst and showed that 1 g of ZnCo 2 O 4 @g-C 3 N 4 @Cu contains 0.53 mmol of Cu.

FESEM and TEM analyses
The morphology of the photocatalyst was probed by FESEM and TEM analyses (Fig. 3).As it is evident from the images, g-C 3 N 4 sheet-like structure accompanied with the cubic ZnCo 2 O 4 can be recognized.The average size of ZnCo 2 O 4 was calculated to be in the range of 25 to 40 nm (Fig. 3c).Furthermore, Fig. 3d showed the lattice fringe at about 0.182 nm, which was consistent with (2 0 0) planes in copper NPs.The average particle sizes of Cu NPs were measured to be 3-5 nm (Fig. 3d).

XPS analysis
XPS analysis was performed to probe the electronic properties and elemental composition of ZnCo 2 O 4 @g-C 3 N 4 @Cu (Fig. S3 †).
In the XPS plot of the photocatalyst (Fig. S3a  in Zn 2+ , arise from the signal peaks at 1021.0 and 1043.9 eV, respectively (Fig. S3e †).As it is evident in Fig. S3f
Compared with the pure g-C 3 N , which has good absorption in the visible light area (Fig. 4a), ZnCo 2 O 4 @g-C 3 N 4 showed enhanced absorptive capacity in the visible light region (Fig. 4b), which originates from the promoted separation rate of e Reaction was accomplished by using g-C 3 N 4 as the catalyst.f Reaction was done using ZnCo 2 O 4 @g-C 3 N 4 as the catalyst.Paper RSC Advances the photogenerated charges. 77,78Incorporation of Cu to the composite could effectually improve the optical capability of the photocatalyst, resulted in an intense absorption of ZnCo 2 O 4 @g-C 3 N 4 @Cu in the visible light region (Fig. 4c), which can be attributed to an increase in the multiple internal scattering of light due to the presence of copper NPs. 79,80Based on the Tauc plot of (ahn) 2 vs. hn (Fig. S5 †), band gap energy for ZnCo 2 O 4 @g-C 3 N 4 @Cu was estimated to be 2.3 eV.These results conrm the high ability of the photocatalyst for enhancing the separation efficiency of the photo-induced electron-hole pairs and so improving the visible-light photocatalytic performance.
C-C cross-coupling reactions over ZnCo 2 O 4 @g-C 3 N 4 @Cu as a heterogeneous Cu-based photocatalyst Arylcyanation reaction of aryl halides.To probe the catalytic activity of ZnCo 2 O 4 @g-C 3 N 4 @Cu for the cyanation reactions, the reaction of iodobenzene with K 4 [Fe(CN) 6 ]$3H 2 O was selected as a typical reaction and the effect of several parameters such as solvents, bases, the catalyst amounts, and light sources was investigated towards the progression of the reaction (Table 1, entries 1-23).The obtained results clearly showed that the superior yield of the desired product was achieved in H 2 O : EtOH (1 : 1), Et 3 N and white LED lamp (20 W) by using 0.7 mol% of the photocatalyst, as the optimum reaction conditions (Table 1, entry 14).Separately studied control experiments under dark reaction conditions and in the absence of the catalyst, did not lead to the desired product (Table 1, entries 24 and 25).Likewise, only a trace amount of the product was acquired in the absence of the base, aer 24 h (Table 1, entry 26).Next, the catalytic potential of g-C 3 N 4 , Cu(OAc) 2 and ZnCo 2 O 4 @g-C 3 N 4 were individually evaluated in the model reaction at the same conditions (Table 1, entries 27-29).No product was attained by using g-C 3 N 4 (Table 1, entry 27) and trace amount of the desired product was acquired over Cu(OAc) 2 (Table 1, entry 28).A similar reaction by applying ZnCo 2 O 4 @g-C 3 N 4 was associated with a poor yield of the product even aer 24 h (Table 1, entry 29).These observations clearly showed that the boosted photocatalytic performance of ZnCo 2 O 4 @g-C 3 N 4 @Cu may be related to collaborative inuence of the photocatalyst components.It is assumed that the visible light harvesting capability of the photocatalyst is directly refers to the appropriate combination of all photocatalytic partners, which can facilitate the electron conductivity in ZnCo 2 O 4 @g-C 3 N 4 @Cu.This phenomenon favors the efficient charge separation and prolongs the lifetime of photo-induced electrons and holes.Interestingly, as the electron reservoir species, Cu NPs can trap the photo-promoted electrons and speed up the reaction under the visible light illumination.
The scope of cyanation cross-coupling reaction over ZnCo 2 -O 4 @g-C 3 N 4 @Cu was extended to various aryl halides with K 4 [Fe(CN) 6 ]$3H 2 O under optimal conditions (Table 2).As illustrated in Table 2, divers iodoarenes (Table 2, entries 1-4), bromoarenes (Table 2, entries 5-12) and chloroarenes (Table 2, entries 13-17) as the most challenging coupling partners, with easier availability and increased affordability compared to the aryl iodides and aryl bromides, participated in the cyanation cross-coupling reaction with K 4 [Fe(CN) 6 ]$3H 2 O to give the desired aryl nitriles in good to high yields.In the cases of 1,4diiodobenzene and 1,4-dibromobenzene, 0.8 mmol of K 4 [Fe(CN) 6 ]$3H 2 O was used as the cyanating agent and 1,4dicyanobenzene was generated (Table 2, entries 3 and 12).It is worth mentioning that in all cases, the reactions were clean and no homocoupling product was detected.

Mizoroki-Heck cross-coupling reaction
Inspired by the promising achievements obtained from the cyanation reaction, in the next step, the photocatalytic applicability of the photocatalyst was evaluated in Mizoroki-Heck reaction under visible light irradiation.
To discover the best reaction conditions, various factors comprising the solvent, base, catalyst loading, and light source were screened to optimize the conditions for the benchmark Mizoroki-Heck reaction of iodobenzene and n-butyl acrylate under visible light irradiation at room temperature (Table 3, entries 1-23).The results of these experiments illustrated that 0.5 mol% of ZnCo 2 O 4 @g-C 3 N 4 @Cu, K 3 PO 4 , EtOH and 20 W white LED can be selected as the optimal reaction conditions  (Table 3, entry 14).The separately conducted control experiments in dark conditions, in the absence of the photocatalyst, and without using base were accompanied with no progress in the reaction, even aer 24 h (Table 3, entries 24-26).Thereaer, the photocatalytic activity of Cu(OAc) 2 , g-C 3 N 4 , and ZnCo 2 O 4 @g-C 3 N 4 were investigated in the model reaction (Table 3, entries 27-29).As observed, trace amount of the product was achieved by using Cu(OAc) 2 as a photocatalyst (Table 3, entry 27).Also, no product was attained by performing the same model reaction using g-C 3 N 4 (Table 3, entry 28).In addition, when the reaction was done over ZnCo 2 O 4 @g-C 3 N 4 , the obtained yield was not satisfying and only 20% of the desired product was generated aer 24 h (Table 3, entry 29).Paper RSC Advances To further study the scope and limitations of Mizoroki-Heck reaction over ZnCo 2 O 4 @g-C 3 N 4 @Cu, various substituted aryl halides were chosen and underwent the reaction with various acrylates under the optimal reaction conditions (Table 4).As indicated in Table 4, a variety of aryl halides including aryl iodides (Table 4, entries 1-6), aryl bromides (Table 4, entries 7-10) and aryl chlorides (Table 4, entries 11-13) as the most challenging halide compounds, which are much more economical and attainable than other aryl counterparts, participates in Mizoroki-Heck reaction to deliver the desired products in good to high yields.In all of these experiments, the reaction medium is clean and any side-product was not detected.
Studies of the heterogeneity of ZnCo 2 O 4 @g-C 3 N 4 @Cu To ascertain whether ZnCo 2 O 4 @g-C 3 N 4 @Cu acts in a real heterogeneous pathway or not, poisoning and ltration tests were performed.For ltration experiment, the model Mizoroki-Heck reaction was done under the optimum conditions and once half of the reaction time has passed, the photocatalyst was isolated from the reaction solution and the reaction was permitted to proceed with no catalyst.Aer continuing the reaction for further 10 hours, no more product was generated, which clearly conrmed that any homogeneous catalyst is not present in the reaction medium (Fig. 5b).ICP-OES analysis of the ltrate was associated with a negligible content of Cu (<0.1% of the total Cu amount).Poisoning test was done by conducting the model reaction of Mizoroki-Heck in the presence of S 8 (0.07 g) as a metal scavenger.As it is apparent, no signicant change in the reaction rate was witnessed in the presence of the scavenger (Fig. 5c).These results showed that ZnCo 2 O 4 @g-C 3 N 4 @Cu has a truly heterogeneous function in this process.

Investigation of a large-scale photocatalytic process
The resulting products in the present work are extremely applicable as the basic building blocks in chemical manufacturing industries.Consequently, to evaluate the practical synthetic applications of this method, the model cyanation and Mizoroki-Heck reactions were separately investigated in a scaled-up procedure (50 times), under optimal conditions.Interestingly, the cyanation reaction proceeded in 8 h, affording 91% yield of the product, while the Mizoroki-Heck reaction was associated with 90% of the desired product aer 6 h.

Studies of the stability and reusability of the photocatalyst in cyanation and Mizoroki-Heck cross-coupling reactions
In heterogeneous photocatalysis, recovering and recycling are very important aspects, particularly for environmental and practical purposes.In this line, the recyclability of the photocatalyst was evaluated for the cyanation and Mizoroki-Heck model reactions under the optimum conditions.As shown in Fig. 6, in both cyanation and Mizoroki-Heck reactions, ZnCo 2 -O 4 @g-C 3 N 4 @Cu could be recovered and reused for at least ve consecutive runs and the nal products were obtained with respectively 84% and 85% yields aer the 5th cycle.These results veried the appreciable durability of the photocatalyst.
ICP-OES analysis of the reused catalyst 5th cycles showed that the recovered catalyst contains 0.50 mmol of Cu per 1 g of the catalyst.This means that the amount of copper leached from the surface of the catalyst is negligible.
These results conrmed the outstanding stability and durability of the photocatalyst for visible-light driven cyanation and Mizoroki-Heck cross-coupling reactions.

Comparative study
The advantages of the presented photocatalyst were compared with previously studied visible light-induced photocatalytic systems in similar coupling transformations (Table 5).While each of these approaches has their own merits, they oen suffer from one or more of the following shortcomings including the strict reaction conditions, use of hazardous solvents, noxious/ expensive/inaccessible light source, large amounts of costly Pd-based photocatalysts, higher reaction temperatures, longer reaction times and lower product yields.These results revealed the superlative photocatalytic activity of ZnCo 2 O 4 @g-C 3 N 4 @Cu, arising from the synergistic optical properties of ZnCo 2 O 4 , g-C 3 N 4 , and Cu.The current approach effectively promoted the cyanation and Mizoroki-Heck reactions through an eco-friendly process, and is highly effective for a broad range of corresponding derivatives.

Photocatalytic mechanism
To insights into a proper reaction mechanism, control experiment was conducted by monitoring the model cyanation and Mizoroki-Heck cross-coupling reactions in the presence of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as a radical scavenger.The results showed that aer involving the radical trapping agent (aryl halide/TEMPO; 1 : 2) the visible light-driven reactions were completely quenched and no product was obtained.Likewise, performing the model coupling reactions in the absence of visible light source was accompanied with no desired product (Table 1, entry 24 and Table 3, entry 24).These results conrmed that such reactions are likely to involve a radical process and light is required to complete the reactions.Having these results in hand and based on the literature review on the heterogeneous photocatalytic cyanation 81,82 and Mizoroki-Heck [59][60][61][62][63] cross-coupling reactions, a proposed reaction mechanism was presented for the visible light-induced cyanation and Mizoroki-Heck cross-coupling reactions (Fig. 8).Upon visible light illumination, both ZnCo 2 O 4 and g-C 3 N 4 were aroused simultaneously and the electrons were excited from the valence band (VB) and transferred to the conduction band (CB) on both ZnCo 2 O 4 and g-C 3 N 4 to produce the photo generated electrons and holes.The photogenerated electrons accumulated on the CB of ZnCo 2 O 4 can easily transfer to the CB of g-C 3 N 4 (due to the low potential energy) with the assistance of the internal electric eld owing to the formation of a p-n heterojunction between ZnCo 2 O 4 and g-C 3 N 4 . 83The existing electrons in the CB of g-C 3 N 4 were simultaneously injected to Cu.These NPs are an effective material for trapping the photogenerated electrons because of their electron reservoir capacity. 84Meanwhile, photogenerated holes in the VB of g-C 3 N 4 can easily immigrate into the VB of ZnCo 2 O 4 .The appropriate transformations of the charge carriers along the p-n heterojunction interfaces of the photocatalyst resulted in the efficient separation of photogenerated electron and hole pairs, which could suitably extend the lifetime of the corresponding excited electrons and holes.Energetic electrons concentrated on the surface of Cu NPs, caused it to undergo oxidative addition with aryl halides.This phenomenon facilitated the cleavage of C-X bonds in aryl halide and formed Ar-Cu-X complex.In Mizoroki-Heck cross-coupling reaction, this step was followed by fast insertion of alkene to Ar-Cu-X complex.0][61][62][63] In the case of cyanation cross-coupling reaction, aer the formation of Ar-Cu-X complex trough an oxidative addition process, cyanide anion of K 4 [Fe(CN) 6 ]$3H 2 O underwent transmetallation reaction to provide a transient organometallic complex intermediate.Finally, aryl nitrile was produced from the complex by reductive elimination and the re-generated catalyst re-entered the catalytic cycle. 81

Experimental
Fabrication of the photocatalyst Synthesis of g-C 3 N 4 nanosheets.The g-C 3 N 4 nanosheets were prepared by slightly modifying the previously reported consecutive polymerization and liquid exfoliation methods. 85Initially, the calcination process of melamine (5 g) was done at 550 °C (5 °C per min) for about 3 h in air.The resultant yellow solid was allowed to reach room temperature.Then, it was completely milled to turn from agglomerated state into a uniform powder.Subsequently, 0.1 g of as-prepared g-C 3 N 4 bulk powder was mixed with deionized water (100 mL) and subjected to ultrasonic treatment for 6 h.Then, the suspension was centrifuged at 3000 rpm for the elimination of the un-exfoliated g-C 3 N 4 and the obtained g-C 3 N 4 nanosheets were collected and dried.
Synthesis of ZnCo 2 O 4 /g-C 3 N 4 .Initially, 0.28 mmol of Co(NO 3 ) 2 $6H 2 O and 0.14 mmol of Zn(NO 3 ) 2 $6H 2 O were dispersed in distilled water (75 mL) for 20 min.Aerwards, a solution of NaOH (2 M) was added dropwise (1 mL per min) into the suspension.The addition of NaOH solution was stopped by the adjustment of the pH solution at 10.Then, the solution was stirred intensively at room temperature for 25 min.In the next step, the obtained suspension was charged with 0.5 g of g-C 3 N 4 nanosheets and reuxed for 1 h.The resultant mixture was centrifuged, washed temporarily with deionized water and dried in a vacuum oven.Eventually, the obtained sample was heated for 2 h in a furnace at 350 °C (5 °C per min) to yield ZnCo 2 O 4 @g-C 3 N 4.
Synthesis of ZnCo 2 O 4 @g-C 3 N 4 @Cu.A suspension containing 1 g of pre-prepared ZnCo 2 O 4 @g-C 3 N 4 in EtOH (30 mL) was sonicated for 30 min.Following this, a solution of Cu(OAc) 2 (2 mmol) in 15 mL EtOH was added drop by drop to the mixture, while constant stirring was applied for 1 h.Subsequently, an aqueous solution of NaBH 4 (30 mL, 0.1 M) was incrementally added to the mixture and stirred vigorously for 3.5 h.The resultant ZnCo 2 O 4 @g-C 3 N 4 @Cu was separated through centrifugation at 1000 rpm, and washed with distilled water (2 × 15 mL) and EtOH (3 × 15 mL), before drying in a vacuum at 60 °C.
General procedure for photocatalytic cyanation reaction using ZnCo 2 O 4 @g-C 3 N 4 @Cu under visible light.ZnCo 2 O 4 @g-C 3 N 4 @Cu (0.7 mol%) was added to a 10 mL Pyrex test tube charged with a mixture of EtOH : H 2 O (1 : 1, 4 mL), aryl halide (1 mmol), K 4 [Fe(CN) 6 ]$3H 2 O (0.4 mmol), and Et 3 N (1 mmol).In all experiments, to prevent any photothermal heating effect, the reaction vial was immersed in a water bath maintained at 25 °C.Thereaer, the reaction container was exposed to a white LED lamp (20 W) at a distance of 10 cm.Aer stirring for an appropriate time mentioned in Table 2, the reaction mixture was diluted with EtOH (5 mL), and the catalyst was separated by centrifugation (1000 rpm), washed with EtOH (2 × 5 mL) and air-dried to prepare for the subsequent reaction process.The solvent of combined organic layer was removed under the rotary evaporation to afford the crude product.The desired pure product was then obtained using a silica gel column chromatography technique (n-hexane : ethyl acetate; 6 : 1).
General procedure for photocatalytic Mizoroki-Heck reaction using ZnCo 2 O 4 @g-C 3 N 4 @Cu under visible light irradiation.ZnCo 2 O 4 @g-C 3 N 4 @Cu (0.5 mol%) was added to a 10 mL Pyrex test tube containing a mixture of olen (1.3 mmol), aryl halide (1 mmol), K 3 PO 4 (2 mmol) and EtOH (4 mL).In all experiments, to prevent any photothermal heating effect, the reaction vial was immersed in a water bath maintained at 25 °C.Thereaer, the reaction container was exposed to a white LED lamp (20 W) at a distance of 10 cm.Aer stirring for an appropriate time mentioned in Table 4, the reaction mixture was diluted with EtOH (5 mL), and the catalyst was isolated by centrifugation (1000 rpm), washed with EtOH (2 × 5 mL) and air-dried to use in the subsequent reaction.Then, the residuals of solvent were removed by vacuum evaporation.The pure product was then afforded using a silica gel column chromatography technique (n-hexane : ethyl acetate; 50 : 1).

Conclusions
In this study, ZnCo 2 O 4 @g-C 3 N 4 @Cu was found as a superb photocatalyst to promote the visible light-driven cyanation and Mizoroki-Heck reactions of a wide range of aryl halides including aryl iodides, aryl bromides, and aryl chlorides (as the challenging class of cross-coupling reactions with few precedents), with K 4 [Fe(CN) 6 ]$3H 2 O and olens, respectively, at room temperature.The synergistic optical effect among ZnCo 2 O 4 , g-C 3 N 4 , and Cu is responsible for the enhanced photocatalytic performance of the photocatalyst.The poisoning and ltration tests were accomplished to verify the actual heterogeneity and durability of ZnCo 2 O 4 @g-C 3 N 4 @Cu under the reaction conditions.The recovered photocatalyst can be readily recycled for at least ve runs while maintaining its catalytic activity and morphology.Applying an economical and benign light source, facile and low-cost photocatalytic approach, no requirement for heat or any additives, scalability of the protocol, aerial conditions, and utilizing an eco-benign solvent are the other important merits of this procedure.It is important to note that the current study represents the rst report of employing a heterogeneous copper-based photocatalyst for the cyanation reactions of various aryl halides with K 4 [Fe(CN) 6 ]$3H 2 O under visiblelight irradiation.

Fig. 5
Fig. 5 Reaction progress vs. the irradiation time for Mizoroki-Heck model reaction in (a) normal conditions, (b) filtration experiment and (c) poisoning test.

Fig. 6
Fig.6Recycling of the photocatalyst in the model cyanation and Mizoroki-Heck reactions under optimal conditions.The reactions were monitored 7 and 6 h for the cyanation and Mizoroki-Heck reactions, respectively.